A transmit/receive (TX/RX) switching system, such as a TDD system, does not allow simultaneous transmission and reception of signals as in a Frequency Division Duplex (FDD) system. This enables a simple design of devices, such as eNBs and UEs. However, fast TX/RX switching is quite a challenge for linear operation of a power amplifier (PA) that is used to amplify the signal to be transmitted, since a variation in load of the system due to switching between TX and RX modes typically causes a swing of a power supply voltage of the PA, which swing, in turn, leads to an undesired modulation of the signal being amplified by the PA for transmission.
FIG. 1 illustrates a conventional circuit for controlling a power amplifier in a TX/RX switching system. As is shown, a PA switch 20 is provided to supply a gate bias terminal of the PA 10 with either a gate bias voltage Vgate (which enables the PA) or a turn-off voltage Voff (which disables the PA). Upon switching of the system from the RX mode to the TX mode, the PA 10 is turned on when the transmission is initiated, by causing the PA switch 20 to couple the gate bias terminal of the PA 10 to the gate bias voltage Vgate, instead of to the turn-off voltage Voff. At this point, the signal to be transmitted is input to the PA 10 for amplification. FIG. 2 is a sequence diagram schematically illustrating operation states of the power amplifier circuit as shown in FIG. 1 during switching of the TX/RX switching system from the RX mode to the TX mode. As illustrated, a swing of a power supply voltage (i.e., Vdd) occurs when the PA 10 is turned on, due to a variation in the load of the TX/RX switching system. This Vdd swing, in turn, brings the PA 10 into non-linear operation, thereby imposing an undesired modulation on the TX signal.
One solution to this problem may be turning on the PA in advance. FIG. 3 schematically illustrates a power amplifier control scheme in the prior art. As illustrated, reception and transmission of data are conducted alternately, with a Guard Period (GP) provided between each reception and transmission for protection of the switching. In particular, a protection time is provided before initiation of each transmission of the data, meaning that the gate bias voltage is supplied in advance to the PA, such that the PA is well turned on and operates in a linear manner when the transmission is initiated. This way, the TX signal would not be affected by the Vdd swing.